In hot-sections of gas turbines and other heat engines, high temperature protective coatings are necessary to protect engine components and improve engine reliability. Alloyed metallic coatings that form protective, slow growing oxide scales such as alumina (Al2O3) and chromia (Cr2O3) have been designed and used as oxidation and corrosion resistant coatings, thus enabling load-bearing components to exhibit extended service lives due to the prevention of fast oxidation and corrosion degradations occurring under high temperature, oxidation and thermal cycling conditions. However, these metallic coatings have a limited temperature capability, with most applications limited to less than 1000° C. Thus large amounts of cooling air are required to reduce component temperatures, even under moderate engine operating temperatures, and engine performance and efficiency are severely limited due to component temperature capability.
In order to increase the temperature capability of engine components, thermal barrier coatings have been developed and applied to component surfaces. Thermal barrier coatings are thin ceramic layers, generally applied by plasma-spraying or physical vapor deposition techniques, used to insulate air-cooled metallic components from high temperature gases. Such coatings are useful in protecting and extending the service life of metallic and ceramic components exposed to high temperatures, such as jet engine turbine blades, vanes and combustors.
Thermal barrier coatings comprised of zirconia-yttria (ZrO2—Y2O3) are well known in the art, wherein yttria typically is present from 7 to 9 weight percent (wt %) (4 to 5 molar percent), and have been widely used in more advanced engine systems. These coatings are typically applied using plasma-spraying or physical vapor deposition in which melted ceramic particles or vaporized ceramic clouds are deposited onto the surface of the component to be protected. Thermal barrier coatings are designed to be porous, with overall porosities generally in the range of 5 to 20%. The porosity serves to reduce the coatings thermal conductivity below the intrinsic conductivity of the dense zirconia-yttria ceramic, and thus retards the flow of heat from the hot gases to the underlying component surface.
As operating temperatures continue to increase, current zirconia-yttria coating conductivities (approximately 2.5 W/m-K) are not acceptable (i.e. too high) for future high performance, low emission turbine engines currently under development. Moreover, the phase and microstructural stability of the current zirconia-yttria coatings remain a significant issue. For example, the destabilization of the zirconia-yttria phases starting at temperatures ranging between 1200-1300° C. may result in the coatings premature spallation. Furthermore, phase destabilization aids in the sintering of a zirconia-yttria coating during high temperature service, thereby decreasing porosity, increasing thermal conductivity and reducing the coatings effectiveness. In summary, the coating and component durability are adversely impacted by the degradation of the coating phase structure and properties associated with higher temperature aging effects. Thus, an improved thermal barrier coating is required for advanced engine applications.